Hitherto, incore structures of a light water reactor, for example, a boiling water reactor, have been made of materials having sufficient corrosion resistance in high temperature and high pressure environment in water and having satisfactory resistance against high temperatures. For example, austenitic stainless steel or a nickel-base alloy has been employed. There is apprehension that members among the incore structures, which cannot easily be changed, deteriorate because the members are subjected to a severe environment as a result of the operation of the nuclear power plant for a long time and influences of irradiation of neutrons. In particular, portions adjacent to welded portions in the incore structure have potential probability of stress corrosion cracking because of sensitization of the materials caused by heat inputs during the welding operations and influences of tensile residual stress.
In recent years, surface modifying techniques for a variety of materials have been developed for preventive maintenance in order to safely operate the nuclear power plant. Among the techniques, techniques for modifying the surface by irradiating the surface of the material with laser beams have been disclosed in, for example, Japanese Patent Laid-Open Publication No. HEI 7-246483 and Japanese Patent Laid-Open Publication No. HEI 8-20689.
That is, the former technique is a laser peening method with which a pulse laser beam oscillated from a pulse laser unit is, through a reflecting mirror, applied to the surface of a member which must be machined (a surface which must be machined). While the position, which is irradiated with the pulse laser beam, is being shifted on the surface which must be machined, the residual tensile stress of the surface which must be machined is converted into compressive stress.
On the other hand, the latter technique is an underwater laser machining method and apparatus using short-pulse laser beams each having a visible wavelength to irradiate the surface, which must be machined and which is immersed in cooling water. Thus, residual stress, cracks or crude is reduced or removed. The foregoing underwater laser machining apparatus and method incorporate an optical fiber to introduce the laser beam from an oscillator to the portion which must be machined. When the optical fiber is employed, the power or the energy for introducing the laser beam in the optical fiber has a limitation. Therefore, the laser beam emitted from the optical fiber must be converged minimally to realize power of light and an energy density required to modify the surface.
Since a general optical fiber for transmitting a power laser beam has a high diffusion coefficient of emitted laser beams, the laser beams must be converged at a large angle. As a result, a converging lens having a very small focal depth must be employed. It leads to a fact that a laser emitting optical system including the optical fibers and the converging lens must precisely be located with respect to the surface which must be machined.
To eliminate the necessity of precisely locating the laser emitting optical system, the inventors of the present invention have applied a preventive-maintenance/repair apparatus in Japanese Patent Application No. HEI 8-256532. The apparatus is capable of eliminating the necessity of precisely locating the optical fibers and the laser emitting optical system. The apparatus has a structure that a laser beam emitted from a laser oscillator disposed on an operation floor or an upper surface of the body of a shroud is transmitted through the air so as to be introduced into the laser emitting optical system. Then, the laser emitting optical system emits laser beams having a high energy density and similar to parallel beams.
The conventional structure of the incore-structure preventive-maintenance/repair apparatus will now be described with reference to FIG. 39.
As shown in FIG. 39, the conventional incore-structure preventive-maintenance/repair apparatus machines an incore structure, such as a shroud 602 installed in a pressure vessel 601 of a boiling water reactor. The incore-structure preventive-maintenance/repair apparatus incorporates a laser-beam source 604 disposed on an operation floor 603; a laser oscillator 605; a light transmission unit 606 having a light-conductive member 606; and a laser emitting head 608 disposed at the leading end of the light-conductive member 606. In the incore-structure preventive-maintenance/repair apparatus, the laser beam L emitted from the laser oscillator 605 is, in the air, transmitted to the laser emitting head 608 so that a portion which must be machined is irradiated with the laser beam L. Therefore, the necessity of precisely locating the laser emitting head 608 can be eliminated. As a result, the laser beam having a high energy density can be used in a preventive-maintenance/repair operation, for example, a laser peening operation, a melting operation, a cladding operation or a welding operation. Thus, the reliability and safety of the nuclear reactor can be improved.
A light transmission apparatus incorporating a plurality of mirrors to transmit laser beams is usually arranged to manually adjust the angles of the mirrors while the positions of the mirrors are being confirmed in a sequential order from a mirror adjacent to the laser unit in the direction of transmission of the laser beam. When the adjustment must be performed in a remote place, for example, a radioactive environment or a high-temperature environment, to which a human being cannot approach, an optical supervisory unit, such as a CCD camera, is disposed at a place which must be supervised. Thus, the mirrors are automatically adjusted while the operator is watching the image.
As described above, the conventional incore-structure preventive-maintenance/repair apparatus shown in FIG. 39 is arranged to machine a portion of an inner wall of the shroud 602 which is the incore structure of the pressure vessel 601 or another incore structure. However, a difficulty is imposed when a vertical welding line (a welding line on the inside of V5) in the lower half of the intermediate body of the shroud or a horizontal welding line (a welding line on the inside of H6a) at the lower end of the inner portion of an intermediate body of the shroud is welded. In the foregoing case, the laser beam must be transmitted to a small cylindrical portion held between the intermediate body of the shroud and the core support plate 609 and having a width of about 30 mm and a depth of about 400 mm. However, the laser emitting head 608 cannot transmit a laser beam to the above-mentioned small cylindrical portion.
When stress is eliminated from the surface of the small cylindrical portion of the intermediate body of the shroud adjacent to the welding line by using the laser beam, when the surface of a sensitized metallographic structure is modified or when a repairing operation is performed by welding, a large incidental angle must be made on the surface which must be irradiated when the points which must be irradiated are scanned to realize an energy density which permits a machining operation using laser beam to be performed.
The foregoing method is structured such that the laser emitting optical system is disposed on the core support plate 609 to irradiate the bottom portion of the small cylindrical portion with laser beams. The foregoing method encounters undesirably reduction in the incidental angle, causing a problem to arise in that a sufficiently high energy density required to perform the machining operation cannot be realized. There arises another problem in that a precise locating mechanism must be provided to scan the points which must be irradiated because the distance which must be irradiated is elongated excessively. As a result, there arises a problem that the preventive-maintenance/repair apparatus becomes too complicated and, therefore, the reliability deteriorates.
When the above-mentioned light transmission apparatus is used to perform a preventive maintenance operation or a repair operation in a nuclear power plant, there arise the following problems in an operation for transmitting and applying laser beams to an incore structure of a nuclear reactor.
Since a sufficiently large working space cannot be permitted in the nuclear reactor and the incore structures are disposed in the core, the light transmission apparatus has a complicated transmission passage. Therefore, the distance for which the laser beam must be transmitted is elongated. As a result, errors are frequently made during the transmission, a large number of places must be supervised and a large number of the optical supervising units, such as CCD cameras, must be provided.
If the environment is changed during transmission of the laser beam in the light transmission apparatus, there is apprehension that the optical axis of the light transmission passage is misaligned owning to the change in the environment. Therefore, the light transmission apparatus must be adjusted to be adaptable to the changed environment as occasion demands.
When the light transmission apparatus must transmit laser beams for a long distance, influences of fluctuations of air are exerted on the laser beam. Moreover, influences of vibrations of peripheral apparatuses are exerted on the laser beam through the transmission mirrors. As a result, the laser beam is undesirably swayed and, therefore, there arises a problem in that the laser beam cannot stably be transmitted to a required transmission point.
If a large number of CCD cameras are provided for the light transmission apparatus, a long time is required to adjust the CCD cameras. Moreover, the electronic elements, such as the CCD camera, cannot be used in an environment in which the high intensity of radioactive rays is shown.
The apparatus of transmitting laser beams disclosed in Japanese Patent Application No. HEI 8-256532 is a preferred apparatus for performing a preventive-maintenance/repair operation of an inner surface of the shroud and a welded optical fiber of an incore structure in the shroud of a boiling water reactor. However, the foregoing apparatus cannot be operated to machine the outside of the shroud body, that is, the outer wall of the shroud body, the inner surface of the pressure chamber and a space called an annulus portion in the form of a small cylindrical shape interposed between baffle plates and having a multiplicity of jet pumps stood erect therebetween. The reason for this matter lies in that no specific mechanism and structure has been disclosed which inserts and brings closer a light conducting pipe, through which the laser beam is transmitted, to the annular space, which must be machined, while the light conductive pipe is moved around impediments, such as the jet pumps. Moreover, no specific mechanism and structure has been disclosed which can efficiently be moved from the core in any one of all 360° horizontal directions.
The inventors of the present invention have found a fact that the method of transmitting laser beams by using the light conductive pipe for transmitting light to the annular portion is able to efficiently perform a preventive-maintenance/repair operation of an incore structure. Thus, a specific machining apparatus and a structure can be provided.
In view of the foregoing, an object of the present invention is to provide a laser-beam emitting head which is capable of efficiently performing a preventive-maintenance/repair operation using laser beams even if a portion which must be machined is a narrow portion, a light transmission apparatus incorporating the laser-beam emitting head and a preventive-maintenance/repair apparatus for an incore structure.
Another object of the present invention is to provide a laser-beam emitting head which is capable of efficiently performing a preventive-maintenance/repair operation of a cylindrical narrow portion held between an intermediate body of a shroud of the inner wall of a shroud body, which is an incore structure, and attached/detached with respect to a core support plate in an underwater environment by using laser beam, a preventive-maintenance/repair apparatus for an incore structure having the laser-beam emitting head and a method of performing the operation.
Another object of the present invention is to provide a preventive-maintenance/repair apparatus for an incore structure which is capable of efficiently and effectively improving stress of a surface layer adjacent to a welding line, modifying the surface of a sensitize (desentization) metal structure and repairing welded portion by using laser beam in an underwater environment such that a welding line of welding lines of the inner wall of the shroud body, which is the incore structure, existing in a cylindrical narrow portion held by an intermediate body of the shroud and the core support plate is made to be the subject.
Another object of the present invention is to provide a light transmission apparatus which is capable of automatically and stably adjusting the optical axis of a light transmission passage formed by combining mirrors from a remote position and an adjustment method.
Another object of the present invention is to provide a light transmission apparatus which is capable of simply and easily adjusting the optical axis to a required position which must be irradiated with light when light is transmitted in the light transmission passage, correcting deflection of the optical axis caused from fluctuation of air and mechanical vibrations of peripheral equipment and stably supplying light to the irradiation position and an adjustment method.
Another object of the present invention is to provide a light transmission apparatus which is capable of automating the adjustment of the optical axis to the final irradiation point and adjusting the optical axis of the light transmission passage from a remote position to enable the apparatus to be used in a portion, such as an incore structure, to which a human being cannot easily approach, and an adjustment method.
Another object of the present invention is to provide a light transmission apparatus which is not require an electronic device, such as a CCD camera which has unsatisfactory radioactive-ray resistance even if light is transmitted to a portion in which the intensity of radioactive rays is high and adjusting the optical axis of a light transmission passage and an adjustment method.
Another object of the present invention is to provide a core preventive-maintenance/repair apparatus adapted to a laser method which is capable of, in an underwater environment, such as cooling water in a reactor pressure vessel, improving stress in a surface layer adjacent to a welding line such that residual tensile stress caused from an influence of heat generated in a welding operation is transformed into compressive stress, modifying the sensitized surface of a metal structure and performing a repair operation of a welded portion such that the surface of a welded structure existing between an outer wall of a shroud body and a baffle plate which are incore structures and in a space partitioned by the inner wall of the reactor pressure vessel.